1. Field of the Invention
The present invention relates to image forming apparatuses, in particular, an image forming apparatus that executes an image stabilization process.
2. Description of the Related Art
Conventionally, an image forming apparatus such as a printer, a copying machine, or an MFP (Multi Function Peripheral) that functions as a printer, a copying machine and the like executes an image stabilization process in order to offer a stable image while suppressing an influence exerted on image quality due to gradual changes in a photoconductor and a developer, changes in environment such as temperature and humidity, and the like.
FIG. 24 is a flowchart showing a flow of a typical image stabilization process. With reference to FIG. 24, specifically, the stabilization process involves a sensor light amount adjusting step (step S10), a maximum density adjusting step (Dmax adjustment) (step S20), a laser light amount adjusting step (step S30), a resist correcting step (step S40) and a tone correcting step (step S50). Hereinafter, brief description will be given of each step.
(1) Sensor (IDC Sensor: Image Density Control Sensor) Light Amount Adjusting Step
The sensor light amount adjusting step refers to a step of adjusting an IDC sensor for detecting an amount of toner attached onto a transfer belt, and this amount corresponds to a density of an image transferred onto a sheet of paper. The IDC sensor is a reflection-type photosensor that detects an intensity of reflected light, and the intensity varies in accordance with an amount of toner attached onto a transfer belt. In the sensor light amount adjusting step, an amount of light emitted from an LED (Light Emitting Diode) serving as a light source is changed such that an output from the IDC sensor based on light reflected from a surface of the transfer belt, where no toner is attached, has a value which falls within a predetermined range. Herein, the surface of the transfer belt, where no toner is attached, is referred to as a “naked surface” or a “bare surface”. In the sensor light amount adjusting step, specifically, the output from the IDC sensor has a value of 4.3 V in the case of the “bare surface”. In a case where the output value decreases as the amount of attached toner increases, the light amount is adjusted such that the output from the IDC sensor has a value which falls within a range of 4.3 V±0.2 V defined as a “predetermined range”.
(2) Maximum Density Adjusting (Dmax Adjusting) Step
In the maximum density adjusting step, control is referred to as control of a maximum amount of attached toner. In order to reproduce multilevel tones, an image forming apparatus changes an “amount of light” from a laser diode (LD) serving as an exposure source and a “density of dots” in image formation. In the maximum density adjusting step, a density of an image is adjusted so as to have a predetermined value in a state that each of the “light amount” and the “dot density” is set at maximum. In the maximum density adjusting step, an amount of toner attached onto the transfer belt, which corresponds to a density of the toner on the transfer belt, is detected in correspondence with image data of a so-called solid image which is reproduced in a state that the light amount is at maximum and the dot density is 100%. Then, image formation conditions such as charging voltage and developing bias are fixed such that the amount of attached toner has a predetermined value.
(3) Laser Light Amount Adjusting Step
The laser light amount adjusting step refers to a step of adjusting an amount of light emitted from the LD to adjust a density per dot. In the laser light amount adjusting step, specifically, the amount of light from the LD is adjusted based on a detected average value of density of image data having a certain dot ratio.
(4) Resist Correcting Step
The resist correcting step refers to a step of detecting and correcting color misregistration due to relative positions of four image forming parts. In the resist correcting step, specifically, a “pattern for detecting a main-scanning misregistration amount” and a “pattern for detecting a sub-scanning misregistration amount” are printed on the transfer belt, and an amount of misregistration of each color is detected from a pattern image scanned by the IDC sensor. Thus, the color misregistration is corrected.
(5) Tone Correcting Step
The image forming apparatus sets an amount of light from the LD and a density of the dots (ON/OFF ratio) in correspondence with a density of image data to be outputted onto a sheet of paper (for example, a density of image data represented by 0 to 255) to output the image data onto the sheet of paper. Therefore, the image forming apparatus stores a relation between the input image data and the output LD light amount or dot density in a form of a table (referred to as a γ table). At the time when the image data is outputted onto the sheet of paper, an LD light amount and a dot density are selected based on the γ table, so that a tone is reproduced. In the tone correcting step, the γ table is corrected such that input image data and a tone characteristic of a printed image establish a predetermined linear relation. In the tone correcting step, a predetermined gradation image is transferred onto the transfer belt, and the IDC sensor reads a density of the gradation image. Thus, the γ table is corrected.
In general, the stabilization process described above is executed occasionally in a warm-up process after turn-on, before execution of the warm-up process, after execution of the warm-up process, upon execution of a printing process, after execution of the printing process, or in such a manner that the printing process under execution is interrupted. That is, the stabilization process is executed when the image forming apparatus intends to execute the printing process to optimize a printing status of the image forming apparatus. Alternatively, change in status based on the printing process is corrected to optimize the printing status of the image forming apparatus. Moreover, when the image forming apparatus returns from a power-saving mode such as a sleep mode, the stabilization process is executed occasionally as in the case of the timing of turn-on.
The stabilization process causes consumption of consumables such as toner, and a waiting time for the printing process. Consequently, it is not appropriate to execute the stabilization process at frequency which is higher than required. However, as an execution frequency of the stabilization process increases, an image to be obtained is improved in image quality. Therefore, various modifications have been made to conditions for and timing of execution of the stabilization process. For example, Japanese Laid-Open Patent Publication No. 11-160921 (hereinafter, referred to as Document 1) discloses the following technique. That is, when processing for forming an image is interrupted due to a trouble such as jamming, determination is made whether to execute a stabilization process in an operation for returning from the interruption. Specifically, Document 1 discloses the technique of determining whether to execute the stabilization process, based on determination conditions including an interruption time and change in environment during the interruption. This technique prevents the stabilization process from being executed more than necessary at the time when the image formation process is interrupted in a short time due to jamming or the like.
Moreover, Japanese Laid-Open Patent Publication No. 2007-072246 (hereinafter, referred to as Document 2) discloses the following technique. That is, in an image forming apparatus that executes a stabilization process at a predetermined time, a stabilization process executing time is determined in accordance with a past count of actually printed sheets of paper kept for each time. This technique prevents a collision of the stabilization process and the printing process, which offers improved convenience to a user.
Further, Japanese Laid-Open Patent Publication No. 2006-234868 (hereinafter, referred to as Document 3) discloses a technique of changing a timing of performing color misregistration correction in accordance with a deviation amount obtained in preceding color misregistration correction. This technique allows execution of a stabilization process at minimum frequency for achieving target quality about the color misregistration.
However, the technique disclosed in Document 1 has the following problem. That is, when a range defined as the interruption is widened so as to cover a power saving mode such as a sleep mode and a power-off state, a threshold value for determining whether to execute the stabilization process can not be set with ease. For example, when a threshold value is set such that the stabilization process is not executed in a case of a long-term interruption, it is possible to suppress the consumption of the consumables and the waiting time, but it is impossible to accomplish an intended object, that is, offering of stable image quality. On the other hand, when the threshold value is set such that the stabilization process is executed even in a case of a short-term interruption, it is possible to offer the stable image quality. However, the execution frequency of the stabilization process increases disadvantageously. As a result, it is impossible to accomplish an object to suppress execution of the unnecessary stabilization process, resulting in consumption of consumables and a waiting time. Further, a requirement for image quality varies for each user.
Moreover, the technique disclosed in Document 2 has the following problem. That is, this technique is not directed to decrease the execution frequency of the stabilization process and, consequently, fails to suppress consumption of consumables. In addition, there is a possibility that the stabilization process is executed in a time zone where the image forming apparatus is not activated so much, but is not executed in a time zone where the image forming apparatus is activated frequently. As a result, there is a possibility that the image forming apparatus fails to offer stable image quality properly.
Further, the technique disclosed in Document 3 has the following problem. That is, if the target quality about the color misregistration does not reach a level required by a user, this technique fails to offer satisfying image quality to the user.
According to the conventional techniques, as described above, it is possible to decrease the execution frequency of the stabilization process. However, it is impossible to offer the satisfying image quality to the user if the execution frequency of the stabilization process is decreased excessively. In actual, consequently, the execution frequency of the stabilization process is set to be large in view of the image quality. As a result, even in a case of adopting the technique capable of decreasing the execution frequency of the stabilization process, advantages of this technique can not be obtained satisfactorily.